1. Field of the Invention
The present invention relates to a process for the production of coarse, scrubbing aggregates of titanium dioxide particles by oxidation of titanium tetrachloride in the vapor phase and use of the aggregates for the prevention of deposit formation in the same production process.
2. Description of the Prior Art
The production of titanium dioxide by vapor phase oxidation of titanium tetrachloride with oxygen or an oxygen-containing gas has been growing in importance compared with older processes in which titaniferous raw materials such as ores and/or slag are digested with mineral acids, preferably sulphuric acid. The reason for this growing importance is that in the vapor phase oxidation, only solid or gaseous reation products are generated and, therefore, no dilute waste acid is obtained whose reconcentration would involve additional process steps and higher operating costs.
During the oxidation of titanium tetrachloride, the temperatures in the oxidation reactor range between 1200.degree. and 2000.degree. C., the variation in temperature being essentially dependent on which oxidation agent is used, viz. air, oxygen or a mixture of both, and whether additional energy is introduced during oxidation, for instance in the form of a booster flame. The stream of hot reaction products (hereinafter referred to as the "reaction mixture") exiting from the oxidation reactor contains titanium dioxide in a very finely distributed form suspended in gaseous constituents.
The freshly formed TiO.sub.2 particles are present at first as very reactive individual particles. During their dwell time in the hot reaction zone, most of these particles grow together to form small aggregates of sizes below 0.15 mm. The individual particles and the small particle aggregates are usable for the manufacture of titanium dioxide pigment.
Part of the small aggregates grow further and partly sinter to form larger, hard and unshapely aggregates of sizes above 0.15 mm. They preferentially form in a layer by deposition of TiO.sub.2 on the surfaces of the hot reaction zone of the oxidarion reactor. This layer is continuously exposed to the abrading action of the hot reaction products passing over it but, on the other hand, is continuously regenerated by the deposition of new TiO.sub.2 particles. In the course of this exchange, larger pieces of this layer are torn off the hot surfaces in an uncontrolled way at irregular intervals and thus undesirably get into the reaction mixture and are carried along with it. These large coarse TiO.sub.2 aggregates are not suitable for pigment production without prior disintegration because their hardness would deteriorate the dispersibility of the pigment and their relatively broad range of sizes would deteriorate optical properties such as tinting strength and hiding power. The formation of such coarse TiO.sub.2 aggregates may at best be reduced in the vapor phase oxidation of titanium tetrachloride, but cannot wholly be prevented.
Titanium dioxide produced in the vapor phase, especially when present in the form of individual particles or small particle aggregates, shows a strong trend--at its temperature on exiting from the oxidation reactor down to a temperature of about 350.degree.C.--to build up on the cooling surfaces especially on the internal walls of the heat exchanger to form tough, firmly adhering layers which do not tend to come off by themselves. This build-up of TiO.sub.2 reduees the heat transfer of the pipe walls and thus the cooling efficiency of the heat exchanger.
In order to prevent the deposition of TiO.sub.2, coarse scrubbing solids of suitable shape and particle size that are chemically inert, i.e., resistant to the hot chlorine-containing reaction gases, are added to the reaction mixture at an appropriate location, i.e., downstream of the oxidation reactor. The purpose of these scrubbing solids, which are added at the lowest possible use level (relative to the TiO.sub.2 generated in vapor phase oxidation), is to keep the internal walls of the heat exchanger permanently free of deposits without causing noticeable material abrasion, especially on those internal surfaces of the cooler that are inclined towards the flow direction of the reaction products. The coarse scrubbing solids must be easy and inexpensive to produce. Since the coarse scrubbing solids preferably have room temperature when added to the reaction mixture, they provide an additional cooling effect.
Numerous substances have been used as the base material for coarse scrubbing solids, such as sand, aluminum oxide, zirconium silicate, inorganic salts, etc.
It is understandable that these foreign substances contaminate the titanium dioxide and must be removed by suitable steps, such as separation in settling chambers or cyclones, or dissolution by water.
To eliminate these drawbacks, processes have been devised in which titanium dioxide is used as scrubbing solids to keep the heat exchanger surfaces free of deposits. This TiO.sub.2 may be produced by the process of digesting titaniferous feedstock in acid. It is more advantageous, however, to use titanium dioxide generated in the vapor phase oxidation of titanium tetrachloride.
In the process described in DE-A-No. 14 42 758, oxidation of titanium tetrachloride can take place in a reaction chamber or a fluid-bed reactor. In the reaction chamber, the reaction constituents are used separately and the coarse TiO.sub.2 scrubbing solids, which are suspended in streams of the reaction constituents or an inert carrier gas, are fed into the oxidation reactor in such a way as to hit surfaces that are accessible to the reaction constituents or their reaction products and to reduce the formation of deposits on these surfaces.
When a fluid-bed reactor is employed, titanium dioxide may be withdrawn from this reactor and, after separation, cooled and, if necessary, subjected to disintegration of particles, so that it may be directed back to the reaction mixture as an abrading material.
The introduction of solid scrubbing agents into the oxidation reactor may keep the interior reactor walls free of deposits, but the process is disadvantageous in terms of energy, because part of the energy required for heating up the reaction constituents is consumed in heating the scrubbing solids. Moreover, part of the freshly generated titanium dioxide adheres to the scrubbing solids that pass through the oxidation reactor thus leading to an undesirable particle growth. This makes it impossible to withdraw particles of a defined particle size fraction from the stream of reaction products; the coarse particle fraction must first be milled or disintegrated by some other means (e.g., particle size reduction by chlorination).
Processes in which abrasively scouring TiO.sub.2 particles are introduced into the oxidation reactor therefore have not been successful in actual practice.
In the process described in U.S. Pat. No. 2,899,278, titanium dioxide produced by vapor phase oxidation of titanium tetrachloride is processed to titanium dioxide pigment by subjecting it to the usual process steps of cooling, separation of pigment from the gaseous reaction products, calcination and milling. The fine TiO.sub.2 dust resulting from intensive milling (e.g., fluid energy milling) initially has no utility, but is treated to yield coarse abrading solids which are directed back to the reaction mixture.
In the process of the above patent, not only TiO.sub.2 particles below a size of 0.15 mm that are usable for pigment production, but also the coarse sintered TiO.sub.2 particle aggregates above 0.15 mm are subjected to the mentioned process steps, thus burdening the plant capacity with material not directly processed to pigment and decreasing the total yield of the TiO.sub.2 production process.
In the process described in U.S. Pat. No. 2,721,626, coarse scrubbing solids of sizes between 0.15 and 6.35 mm, e.g., aggregated TiO.sub.2 particles, are admixed with the hot reaction mixture and after cooling, separated from the reaction products in the gaseous phase in a suitable separating apparatus and again used as scrubbing solids. The dry separation of the coarse scouring TiO.sub.2 aggregates larger than 0.15 mm from the finely particulated titanium dioxides in devices such as cyclones, settling chambers, air classifiers, etc. is technically unsatisfactory which means the separating effect is insufficient. As a result, the coarse TiO.sub.2 aggregates recycled as scrubbing solids in the heat-exchanger always contain a percentage of fine-sized TiO.sub.2 which percentage constantly and undesirably increases upon recirculation due to the steady abrasion of coarse TiO.sub.2 aggregates in the heat exchanger. Consequently, the separating effect of the apparatus further deteriorates. It is not feasible to control the particle size distribution by correspondingly increasing the production of coarse TiO.sub.2 aggregates as the size distribution of the TiO.sub.2 aggregates exiting from the oxidation reactor is essentially determined by the reaction conditions prevailing in the oxidation reactor. In this process, the coarse TiO.sub.2 scrubbing solids are obtained by calcining the filtered fine-sized TiO.sub.2 at 600.degree.-1000.degree. C. The portion transformed into coarse aggregates during this calcination is lost for the direct production of pigment. Hence, the overall efficiency of the process is reduced.